Conventional hot-wire type air flow meters having a bypass passage are known as disclosed in U.S. Pat. Nos. 3,314,290 and 4,709,581, Japanese laid-open patent application 57-105551 and Japanese laid-open utility model application 56-163668.
According to the above-disclosed conventional air flow meters, an outlet of a bypass passage is opened toward a throttling portion formed within an intake conduit in such a way that intake air flowing through the intake conduit is introduced into the bypass passage by the pressure difference that occurs between an inlet portion of the bypass passage and the throttling portion. The flow rate of air introduced within the bypass is detected by a hot-wire sensor provided in the bypass passage. When this kind of air flow meter is used for measuring the flow rate of intake air of an internal combustion engine for an automobile, the air flow meter is provided between an air cleaner and a throttle valve. For example, the air flow meter may be directly attached to the air cleaner casing. In such a case, however, an air flow of the introduced air is disturbed within the air cleaner casing before being monitored by the air flow meter. Therefore, an air flow near an outlet of the air cleaner casing becomes unequally distributed, and therefore the air flow immediately upstream of the air flow meter is similarly unequally distributed. The same phenomenon occurs when an intake conduit upstream of the air flow meter is bent near the air flow meter, or when a filter element of the air cleaner is soiled.
When there is unequal air flow upstream of the air flow meter, the flow rate of air introduced into the bypass passage is different from the average flow rate of intake air. As a result, an inaccurate flow rate is detected by the hot-wire sensor because of the above-described unequal air flow.